Upper air ultraviolet germicidal equipment is designed to provide air disinfection within an occupied space. The louvered upper air units are wall or ceiling mounted above the "human zone" in the space, generally at 7 feet or higher.
The usual UV-C 253.7 nanometer (nm) wavelength emitted by the germicidal light bulbs is directed out of the unit by the louvers and into the layer of air at or above the mounting height of the unit. Natural convection currents in the space cause all the air in the room to pass through this irradiation zone and become sanitized.
When viewed directly, 253.7nm light is extremely harmful to the human eye. By mounting the upper air UVC units well above eye level and with the louvers blocking a view from below, it is unlikely the occupants will be able to "see" the light bulbs in the unit.
Normal room occupancy, while the air in the space is continually disinfected, is the principal advantage of upper air UVC. Open UVC fixtures with exposed light bulbs can be used to disinfect the air in a room, but they cannot operate while people are in the space.
While direct exposure to the human eye is the main concern with 253.7nm light, minimizing skin exposure is also important. If the louvers are working properly and directing the irradiation into the upper air zone, the chances of significant exposure to the skin is unlikely.
This diagram shows a typical UV-C elongated oval distribution from the upper air unit into the room. The intensity of the UV irradiation is measured in microwatts per square centimeter (μW/cm²). Within the germicidal zone, the highest intensity is within a few feet of the upper air fixture. As the distance increases, the intensity declines but remains above 10μW/cm², a level that can still be effective at disinfecting the air. Notice that within a few inches below the germicidal zone, the intensity drops to 0.2μW/cm². This level is considered safe for the occupants.
For a detailed study on the effects of UV-C irradiation on people in a space equipped with upper air disinfecting units, read this article from National Institute of Health:
Upper air UVC equipment is an effective way to provide air disinfection in a space when it is occupied. The principal advantage is continuous, silent operation while allowing the space to be used normally. Upper air fixtures are usually designed for rooms that range from 100 to 600 sq. ft. Larger rooms can be equipped with multiple units.
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